The study of human cells defective in repairing damaged DNA was extended, with the rationale that DNA repair-deficient cells are more susceptible to the adverse effects of carcinogens (cell killing, mutagenesis, sister chromatid exchange, and malignant transformation) than their repair-proficient counterparts. A group of human tumor (19) and SV40-transformed (7) strains deficient in the repair of 0-6-methylguanine (0-6-MeG, a modified DNA base made by certain methylating agents) was identified earlier in this project. Such strains are called Mer-. Transformation by SV40, Rous sarcoma virus, adenovirus, or Epstein-Barr virus produces Mer- strains. A 22,000 MW protein, present in 13 Mer+ strains but not in 16 Mer- strains, demethylates 0-6-MeG in DNA, thereby repairing this damaged base and producing guanine. After the reaction, the methyl group remains bound to protein, presumably to the 0-6-MeG-DNA methyl-transferase itself. Using methyl group transfer as a stoichiometric measure, Mer+ strains contain on the average 60,000 methyltransferase molecules per cell. A group of 5 Mer+ cell strains, sensitive to cell killing by MNNG, was termed Mer+ Rem-. This group was found able to repair approximately one-third as much 0-6-MeG as Mer+ Rem+ cell strains. Two groups of Mer- Rem+ cells were identified. Such strains fail to repair 0-6-MeG but are resistant to cellular inactivation by MMNG. This shows that 0-6-MeG (if lethal to Mer- Rem- strains) is not lethal to Mer- Rem+ strains. Mer- Rem- and Mer- Rem+ strains are equally sensitive to inactivation by chloroethylnitrosoureas, indicating that while repair of 0-6-MeG may not be necessary for survival in Mer- Rem+ strains, repair of 0-6-chloroethylguanine (an adduct that leads to DNA:DNA crosslinks) probably is necessary. Finally, data obtained so far indicate that human interferons Alpha and Beta inactivate Mer- Rem- tumor strains while Mer+ Rem+ human tumor strains are more resistant to such treatment, indicating an association between defective repair of 0-6-MeG and sensitivity to interferons.